THE global economy seemed bound for a shaky start in 2013, with fiscal cliffs lurking in America and a chronic crisis in Europe. Yet things could have easily turned out worse. America’s politicians bungled about, but avoided disaster. Despite a messy bail-out in Cyprus, government-bond yields across the troubled euro-zone periphery are falling. The world has been mercifully free of seismic catastrophes and soaring oil prices.

That good fortune has not prevented a broad slowdown. Global growth sank below 2.5% in the second half of 2012. A small rebound at the start of 2013 appears to have faded. First-quarter GDP reports disappointed in America and China. Unemployment in Europe ticks relentlessly upwards. And in April an important index of global economic activity sank to its lowest level since last October, suggesting …